Motor

ABSTRACT

A small DC motor includes a motor frame having a cylindrical portion, the cylindrical portion having a constant thickness and having a cross section in a shape that comprises four sides and connecting portions, each of the connecting portions connecting adjacent two of the four sides and being located inward from a corresponding corner in a quadrangle comprising the four sides; and a magnet having a circumferential surface on an inside thereof and having conformable contact with the motor frame on an outside thereof.

This application is a continuation of application Ser. No. 12/659,481,filed Mar. 10, 2010, which is a continuation of application Ser. No.12/320,688, filed Feb. 2, 2009, which is a continuation of applicationSer. No. 11/441,191, filed May 26, 2006.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a small DC motor which is reduced involume and is made to be easily mounted.

2. Background Art

In DC motors which are used in laser printers, inkjet printers and otherelectrical equipment, there exists a demand that motors be reduced insize while the properties thereof are maintained.

Due to this, the external shape of a motor is made smaller in such astate that a magnetic constituent portion (outside diameter andlamination thickness of an armature core) of a rotating element whichdetermines the properties of the motor is maintained by changing theshape of a magnetic constituent portion (magnet, motor frame) of astationary element.

For a DC motor to rotate, since a ratio of the number of magnetizedmagnetic poles (N: a positive arbitrary number) to the number of slots(M: a positive arbitrary number) basically becomes 2N:3M, the number ofmagnetized magnetic poles of a magnet is 2 poles, 4 poles, 6 poles, 8poles, . . . . Generally, DC motors are formed into oval and cylindricalshapes, and in the event of a motor of an oval shape, a magnet ismagnetized to 2 poles, whereas in the event of a motor of a cylindricalshape, a magnet is magnetized to 2 to 2N poles. For the cylindricalmotor to be reduced in size, the grade (maximum energy product:(BH)_(max)) is increased, the thickness of the magnet is reduced and theoutside diameter of a core is reduced while the properties of the motorare maintained. However, in the event that the thickness of the magnetis changed, it is extremely difficult, in most cases, to maintain themotor properties only by upgrading the grade of the magnet.

In addition, in the motors which are required to be reduced in size,motor frames are formed as small as possible due to the necessity ofnarrowing the accommodation space. Due to this, in many cases, motorframes are formed into cylindrical and oval shapes so as to match theshapes of rotors, and also in motor frames of a type in which fieldmagnets are mounted thereon, the cylindrical and oval shapes aresimilarly used.

A related art is disclosed in JP-A-07-059322 (the term “JP-A” as usedherein means an “unexamined published Japanese patent application”)below which attempts to realize the reduction in the external size of amotor and easy mounting thereof while maintaining the properties of amotor. By forming a motor frame into a quadrangular shape having thesame number of corner portions as the number of magnetized magneticpoles of a magnet, the reduction in size of a motor is enabled withoutchanging the external shape of an armature core and furthermore withoutreducing the thickness of central portions of the poles of the magnet soas to maintain the properties of the motor.

FIG. 8 is a cross sectional view of a small DC motor described inJP-A-7-059322.

A small DC motor 100 in FIG. 8 comprises a quadrangular cylindricalmotor frame 101 having a substantially square cross section, a magnet102 accommodated inside the motor frame 101 and having a circularcylindrical space therein which has four magnetic poles which aremagnetized alternately to N pole and S pole in such a manner thatcenters of the magnetic poles come to lie in corner portions of thequadrangular cylindrical motor frame, respectively, a rotor 103 which isaccommodated in the circular cylindrical space in the magnet 102 via aradial gap, and a feeding terminal (not shown) which is drawn out of oneside of the quadrangular cylindrical motor frame 101 so that the motoris placed horizontally on an printed circuit board (not shown) for use.

In the small DC motor 100 disclosed in JP-A-07-059322 which has thefour-magnetic-pole field magnet 102 which is magnetized to have the fourmagnetic poles which are magnetized alternately to N pole and S pole,the motor frame which holds the field magnet 102 is formed so as to havethe substantially square cross section, and the field magnet 102 ismagnetized so that the centers of the magnetic poles come to lie in thecorners of the square. A magnet used as the field magnet 102 is made upof a plastic magnet and is molded together with the quadrangularcylindrical motor frame 101.

According to this configuration, since the motor takes the angular orquadrangular prism shape, the motor can be fixed to a printed circuitboard or the like by being bonded thereto by means of a adhesive doublecoated tape. Since the thickness of the magnet 102 at the portionsthereof which lie in the corners of the quadrangular shape is increased,an actual permeance coefficient is increased.

In the small DC motor described in JP-A-07-059322, however, since themagnet is filled even to the four corners of the motor frame, thethickness of the magnet is such as to be more than what is needed by anactual driving property. Namely, when viewed in a radial direction fromthe center of rotation, the thickness of the magnet becomes thickest atthe portions of the magnet which correspond to the four corner portionsof the motor frame and exceeds a magnet thickness where a sinusoidalproperty needed by a magnetic property (a magnetic flux densityproperty) is formed. Due to this, the magnet portions which correspondto the four corner portions of the motor frame can be cut by such anextent that there is caused no problem with the magnetic property.

In addition, since an advantage is said to be provided that when fixinga motor frame, the motor frame can easily be fixed in the event that themotor frame has plane sides, the advantage is understood to be providedoverall in the event that part of each of the original plane sides ofthe motor frame remains intact. The aforesaid related art example hasproblems with these points.

SUMMARY OF THE INVENTION

In view of the points raised as the problems, an object of the inventionis to provide a small DC motor which is easy to be mounted and isreduced in size without lowering the driving property.

With a view to accomplishing the object, the following solutions will beprovided.

(1) There is provided a small DC motor including a motor framecomprising a cylindrical portion, the cylindrical portion having aconstant thickness and having a cross section in a shape that comprisesfour sides and connecting portions, each of the connecting portionsconnecting adjacent two of the four sides and being located inward froma corresponding corner in a quadrangle comprising the four sides; and amagnet having a circumferential surface on an inside thereof and havingconformable contact with the motor frame on an outside thereof.

(2) There is provided a small DC motor as set forth in (1) above,wherein each of the connecting portions has arc-shape.

(3) There is provided a small DC motor as set forth in (1) above,wherein each of the connecting portions has arc-shape of ratio relativeto a radius of the circumferential surface of the magnet.

(4) There is provided a small DC motor as set forth in (1) above,wherein each of the connecting portions has a straight line-shape.

(5) There is provided a small DC motor as set forth in (1) above,wherein the cross section of the motor frame has a shape with 2(n+1)sides and angles, and n represents a positive integer.

(6) There is provided a small DC motor as set forth in (1) above,wherein the magnet comprises portions magnetized to different magneticpoles alternately in one direction along the circumferential surface.

(7) There is provided a small DC motor as set forth in (6) above,wherein the portions magnetized are separated from one another.

(8) There is provided a small DC motor as set forth in (6) above,wherein the portions magnetized to different magnetic poles have contactwith one of the connecting portions of the motor frame.

(9) There is provided a small DC motor as set forth in (1) above,wherein a ratio of a maximum width (L2) to a minimum width (L1) in aradial direction of the magnet is set in the range of 2.0≦L2/L1≦3.0.

(10) There is provided a small DC motor as set forth in (3) above,wherein the ratio of the arc-shape of the connecting portions to theradius of the circumferential surface of the magnet ranges from 65% to85%.

(11) There is provided a small DC motor as set forth in (1) above, whichcomprises an armature assembly rotatably disposed within the motorframe, wherein the armature assembly comprises: an armaturewindings-molded element of a cylindrical coil wounded by a magnet wire;and an inner yoke comprising a cylindrical portion fixed in such amanner to face the armature windings-molded element.

(12) There is provided a small DC motor as set forth in (1) above, whichcomprises an armature assembly rotatably disposed within the motorframe, wherein the armature assembly comprises: an armaturewindings-molded element of a cylindrical coil wounded by a magnet wire;and a cylindrical movable back yoke having contact with an inner side ofthe armature windings-molded element.

Since a small DC motor of the invention is made such as to include amotor frame having a motor frame comprising a cylindrical portion, thecylindrical portion having a constant thickness and having a crosssection in a shape that comprises four sides and connecting portions,each of the connecting portions connecting adjacent two of the foursides and being located inward from a corresponding corner in aquadrangle comprising the four sides; and a magnet having acircumferential surface on an inside thereof and having conformablecontact with the motor frame on an outside thereof, the shape of themagnet can be made smaller in size while maintaining the necessarymagnetic property, and in association with this, the shape of the motorframe can be made easier to mount and smaller in size.

Even in the event that the cross section of the motor frame has a shapewith 2(n+1) sides and angles, the intended advantage is provided. Inthis regard, n is a positive integer.

Since, as viewed in a radial direction thereof, a ratio of a maximumwidth (L2) to a minimum width (L1) of the magnet is set in the range of2.0≦L2/L1≦3.0, the shape of the magnet can be made smaller in size whilemaintaining the necessary magnetic property.

By incorporating the field magnet configuration of the invention in acoreless motor or slotless cored motor, those motors can be made smallerin size while increasing torque to be generated.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention disclosed herein will be understood better with referenceto the following drawings of which:

FIGS. 1A, 1E and 1I are one side views, FIGS. 1B, 1F and 1J aretransverse sectional views, FIGS. 1C, 1G and 1K are longitudinal views,and FIGS. 1D, 1H and 1L are another end views, which explain features ofa small DC motor of the invention through comparison with a relatedexample;

FIG. 2A is a sectional view which explains features of a motor frame ofthe invention through a comparison with the related example and FIG. 2Bis an enlarged vertical longitudinal sectional view which explainsfeatures of a magnet of the invention through a comparison with therelated example;

FIGS. 3A1 to 3E1 are end views and FIGS. 3A2 to 3E2 are transversesectional views, which explain various embodiments of the invention, andFIG. 3F1 is an end view and FIG. 3F2 is a transverse sectional view,which explain embodiment of the related art;

FIG. 4 is a torque characteristic graph by ratios of L2:L1 of themagnets of the invention;

FIG. 5 is a characteristic graph of magnetic property and mass (cost) byratios of radius of corner R to inside radius of the magnets of theinvention;

FIG. 6A is a longitudinal sectional view and FIG. 6B is a transversesectional view, which illustrate a coreless motor to which the magnetconfiguration of the invention is applied;

FIG. 7A is a longitudinal sectional view and FIG. 7B is a transversesectional view, which illustrate a slotless cored motor to which themagnet configuration of the invention is applied; and

FIG. 8 is a transverse cross section of a small direct current (DC)motor described in JP-A-7-059322.

DETAILED DESCRIPTION OF THE INVENTION

Exemplary embodiments of the invention will be described in detail belowbased on the accompanying drawings. However, it is to be understood thatthe invention is not intended to be limited to the specific embodiments.

(Embodiment 1)

FIGS. 1A to 1L illustrate explanatory drawings which describe featuresof a small DC motor of the invention while comparing it to relatedcounterparts.

FIGS. 1A to 1D are drawings which show the configuration of a relatedgeneral DC motor which utilizes ferrite magnets, in which FIG. 1A is aone side view as seen in a direction indicated by arrows a in FIG. 1C,FIG. 1B is a transverse sectional view taken along a line b-b in FIG.1C, FIG. 1C is a sectional view taken along a line c-c in FIG. 1B, andFIG. 1D is a another side view as seen in a direction indicated byarrows d in FIG. 1C.

In FIGS. 1A to 1D, a construction is adopted in which two magnets 105being magnetized to N pole and S pole in a rotating direction and eachhaving an arc-like cross section are axisymmetrically provided with gapsprovided therebetween in such a manner as to follow a side wall of acircular cylindrical motor frame 106.

When adopting the construction, in an example, with a rotor having adiameter of 23 mm, the diameter of the motor frame becomes 35.8 mm.

FIGS. 1E to 1H are drawings which show the configuration of a related DCmotor which utilizes neodymium magnets, in which FIG. 1E is a one sideview as seen in a direction indicated by arrows e in FIG. 1G, FIG. 1F isa transverse sectional view taken along the line f-f in FIG. 1G, FIG. 1Gis a sectional view taken along a line g-g in FIG. 1F, and FIG. 1H is aanother side view as seen in a direction indicated by arrows h in FIG.1G.

In FIGS. 1E to 1H, a construction is adopted in which four magnets 111being magnetized to N pole or S pole in a rotating direction and eachhaving an arc-like cross section are axisymmetrically provided in such amanner as to follow a side wall of a circular cylindrical motor frame112. By changing grades of magnets from the ferrite magnets((BH)max(BaO.6Fe₂O₃)..13 (J/m³×10³), (BH)max(SrO.6Fe₂O₃)..17 (J/m³×10³))shown in FIG. 1A whose maximum energy product ((BH)max) is relativelysmall to the neodymium magnets ((BH)max(Nd₂Fe₁₄B)..180 (J/m³×10³)) inthis example whose maximum energy product ((BH)max)) is large, with arotor of the same diameter, the diameter of the motor frame can bereduced. When adopting the construction, in an example, with a rotorhaving a diameter of 23 mm, the diameter of the motor frame becomes 32mm.

FIGS. 1I to 1L are drawings which show the configuration of a small DCmotor of the invention which utilizes neodymium magnets, in which FIG.1I is a one side view as seen in a direction indicated by arrows i inFIG. 1K, FIG. 1J is a transverse sectional view taken along a line j-jin FIG. 1K, FIG. 1K is a sectional view taken along a line k-k in FIG.1J, and FIG. 1L is an another side view as seen in a direction indicatedby arrows l in FIG. 1K.

In a small DC motor 1 shown in FIGS. 1I to 1L, a construction is adoptedin which four magnets 2 being magnetized to N pole or S pole in arotating direction and each having an arc-like cross section areaxisymmetrically provided in such a manner as to follow a side wall of acylindrical motor frame 3 to thereby be formed into an annular shape.

The motor frame 3 has a cylindrical portion 4, the cylindrical portion 4having a constant thickness and having a cross section in a shape thatcomprises four sides and connecting portions, each of the connectingportions connecting adjacent two of the four sides and being locatedinward from a corresponding corner in a quadrangle comprising the foursides and an end plate portion 5 having therein an opening through whicha shaft 7 is passed. In particular, in the event of this example, theconnecting positions have a shape which is rounded by an arc whichconstitutes part of a circle concentric with the center of the shaft 7.

An end cap 6, which is made of resin, is fitted in an open end of thecylindrical portion 4 of the motor frame 3. The end cap 6 has an openingthrough which the shaft 7 is passed, and brushes, adapted to be broughtinto contact with a commutator that is to be provided on the shaft 7,and a terminal portion thereof are fixedly press fitted in the end cap.

The magnets 2 has a circumferential surface on an inside thereof and hasconformable contact with the motor frame on an outside thereof.

When this construction is adopted, in an example, with a rotor having adiameter of 23 mm, a shortest outside diameter of the motor framebecomes 28.5 mm.

In addition, the material of the magnet 2 to be adopted does not differfrom that of the related DC motor shown in FIG. 1( e) which utilizes theneodymium magnets.

When considering what has been described heretofore, according to thesmall DC motor 1 of the invention, by adopting the configuration inwhich the small DC motor 1 includes the motor frame 3 comprising thecylindrical portion 4, the cylindrical portion 4 having a constantthickness and having a cross section in a shape that comprises foursides and connecting portions, each of the connecting portionsconnecting adjacent two of the four sides and being located inward froma corresponding corner in a quadrangle comprising the four sides; andthe magnet having a circumferential surface on an inside thereof andhaving conformable contact with the motor frame on an outside thereof,the small DC motor 1 of the invention is to have features in which thecross-sectional area of the motor frame 3 can be minimized whilemaintaining a magnetic property (a sinusoidal property according torotation) which is necessary to maintain the driving property, and inwhich since planes including sides made up of straight lines are formedon side walls of the motor frame 3, when viewed in cross section, themotor frame 3 can easily be mounted.

(Shapes of Magnets)

Among constituent parts which make up a DC motor, magnets are mostexpensive parts and hence need to be minimized in mass while maintainingthe motor properties.

The magnets of the invention will be described while compared to therelated magnets disclosed in JP-A-07-059322.

FIGS. 2A and 2B illustrate explanatory drawings which explain thefeatures of the motor frame and the magnets of the invention throughcomparison with those of the related example.

To be specific, the drawings illustrated therein are comparison drawingsamong the related magnets described in JP-A-07-059322 (refer to FIGS.3F1 and 3F2), the magnets of the invention illustrated in FIG. 1J (referto FIGS. 3C1 and 3C2), and magnets for a representative motor framehaving an irregular octagonal shape, which will be described later on(refer to FIGS. 3B1 and 3B2).

FIG. 2A illustrates the respective transverse cross sections of themotor frames, and FIG. 2B illustrates the enlarged vertical section ofone magnetic pole of the magnets.

Firstly, the shapes of the magnets will be described.

In FIG. 2B, when the cross sections of the respective magnetic poles ofthe magnets are aligned with each other on their inside surfaces, thecross section of the related magnet described in JP-A-07-059322 takes ashape f4 indicated by a solid line, the cross section of the magnet ofthe invention shown in FIG. 1J takes a shape c4 indicated by a dottedline, and the cross section of the magnet for the irregular octagonalside frame takes a shape b4 indicated by an alternate long and shortdash line.

The shapes b4 and c4 according to the invention are such as to resultwhen the corners of the related shape f4 deform in direct of the insidesurface of the magnet.

When the shapes resulting when the corners of the related shape f4deform in direct of the inside surface of the magnet are expressed byradial widths from the center of the shaft, in the event of the shapeb4, a ratio of a maximum width L2 b to a width L1 b (=L1 c) at both endsin the rotating direction is set in the range of 2.0≦L2 b/L1 b≦3.0, andin the event of the shape c4, a ratio of a maximum width L2 c to aminimum width (L1 c) at both ends in the rotating direction is set inthe range of 2.0≦L2 c/L1 c≦3.0.

(Ratio)

Next, the reason that the minimum width:maximum width ratio is set inthe range of 1:2 to 1:3 will be described.

Measured data shown in Table 1 and Table 2 below were acquired by usingthe small DC motor of the invention shown in FIGS. 1I to 1L. A motor wasused which was made up of a rotor having a diameter of 23 mm and a motorframe having a shortest diameter of 28.5 mm.

FIG. 4 is a torque characteristic graph by magnet's L2:L1 ratios. Datashown in FIG. 4 are data measured by using magnet shown in FIG. 1J andare summarized in Table 1 below.

TABLE 1 Torque Characteristics by Magnet's L2:L1 Ratios Starting TorqueCogging Torque L2/L1 (mN · m) (mN · m) 0.0 0.5 68 30 1.0 76 24 1.5 88 182.0 95 23 2.5 96 26 3.0 97 27 3.5 98 29 4.0 100 30 4.5 100 31 5.0 100 305.5 100 29

In the torque characteristics shown above, an L2 to L1 ratio range of2.0≦L2/L1≦=3.0 corresponds to a region where the starting torque isrelatively large and the cogging torque is relatively small.

Next, the shape to which the corner of the magnet illustrated in FIG. 1J(refer to FIGS. 3C1 and 3C2) is rounded will be described.

FIG. 5 is a characteristic graph which shows magnetic property and mass(cost) by ratios of radius of corner R to inside radius of the magnet ofthe invention. Data shown in FIG. 5 are data measured by using themagnet illustrated in FIG. 1J and are summarized in Table 2 below.

TABLE 2 Magnetic Property and Mass (Cost) by Ratios of radius of CornerR to Inside Radius of the Magnet Ratios (%) of Starting Corner R toTorque Cost/Mass Inside Radius (mN · m) (yen) 0 13.5 100 164 25.5 100158 37.5 100 149 49.0 98 138 61.0 97 123 74.0 96 108 85.0 95 89 97.0 8866 110.0 76 16

A ratio of radius of corner R to inside diameter of the magnet (namely,the radius of curvature of the corner relative to the inside diameter(radius) of the magnet) is set in a range of 65% to 85% as a range whichsatisfies conditions of realizing the facts that the magnetic propertyby ratios of radius of the corner R (the length of a radius of an arcwhich forms an outer curved surface of the magnet) to inside radius (thelength of a radius of an arc which forms an inner curved surface) of themagnet resides in a relatively high range, while the mass property byratios of radius of the corner R to inside radius of the magnet, thatis, a property in which mass is expressed by cost resides in arelatively inexpensive range.

As a result, in the event of the shape in which the corner of the magnetis rounded (R), one or both of the following two conditions are set:

-   (1) A condition that the ratio of the width L2 to the width L1 of    the magnet is set in the range of 2.0≦L2/L1≦3.0; and-   (2) A condition that the ratio of radius of the corner R to the    inside radius of the magnet is set in the range of 65% to 85%.

On the other hand, in the event of the magnet (refer to FIGS. 3B1 and3B2) for the irregular octagonal motor frame, the corner is formed intoa shape which results after being cut straight by a straight line asshown by the shape b4 in FIG. 2B. While the inclination of the straightline can be basically set arbitrarily, the inclination thereof is set tothe inclination of a tangent which is tangent to an arc drawn with aradius extended from the center of the arc which forms the inner curvedsurface of the magnet, an inclination at a predetermined angle from anouter straight portion of the magnet when viewed in cross section or aninclination at an angle which matches the polygonal shape of the motorframe.

In the event of the shape resulting when the corner is cut straight bythe straight line, the condition is necessary that the ratio of thewidth L2 to the width L1 of the magnet is set in the range of2.0≦L2/L1≦3.0.

In addition, the small DC motor of the invention may be such that themagnets are magnetized alternately to different magnetic poles in onedirection along the circumferential surface, or the magnetized portionsof the magnets may be separated from each other. Further, the portionsof the magnets which are magnetized to different magnetic poles may haveportions which have conformable contact with the connecting portions ofthe motor frame.

(Shapes of Motor Frames)

FIG. 2A is a drawing which shows external shapes of the cross sectionsof the motor frames.

In the figure, the related motor frame described in JP-A-07-059322constitutes a shape f3 indicated by a solid line, the motor frame of theinvention shown in FIG. 1J constitutes a shape c3 indicated by a dottedline, and the irregular octagonal motor frame of the inventionconstitutes a shape b3 indicated by an alternate long and short dashline. When compared to the shape f3 of the related example, it is seenthat the shapes c3 and b3 of the invention constitute shapes in whichcorners thereof are largely cut, reducing a space that is occupied bythe motor frames, respectively. In addition, since straight sides areformed on both sides of the arc, a stable mounting, free from vibration,is possible.

In addition, in the small DC motor of the invention, each of theconnecting portions may have arc-shape, arc-shape of ratio relative to aradius of the circumferential surface of the magnet, or the straightline-shape.

Further, the cross section of the motor frame 3 may have a shape with2(n+1) sides and angles, and n represents a positive integer. Accordingto this configuration, the basic shape (the shape resulting before thecorner portions are collapsed) of the cylindrical portion 4 of the motorframe 3 can have, for example, a quadrangular shape, a hexagonal shape,an octagonal shape, . . . .

(Embodiment 2)

The magnets and motor frames of the invention will be described throughcomparison with those of the related example.

FIGS. 3A1 to 3E1 and FIGS. 3A2 to 3E2 illustrate explanatory drawingswhich depict various embodiments of the invention. Note that in FIGS.3A1 to 3E1 and FIGS. 3A2 and 3E2, drawings which have the drawing numbersuffixed by “1” are drawings which illustrate end plates of the motorframes. In other words, they are drawings which illustrate respectiveexternal shapes of the motor frames. Drawings which have the drawingnumber suffixed by “2” are drawings which illustrate respective crosssections of the motor frames.

In FIGS. 3A1 and 3A2, when viewed in cross section, a motor frame 3 atakes basically a round shape, and a magnet 2 a is formed into a shapewhich secures a permeance of the magnet which is necessary to maintainthe motor properties, suppresses the width of the magnet where themagnetic poles are changed over to a minimum width, enables a ratio ofL1:L2, that is, a ratio of a radial thickness L1 of a portion (athinnest portion) where the magnetic poles are changed over to a radialthickness L2 of a central portion (a thickest portion) of the magneticpole to be in the range of 1:2 to 1:3, and leaves straight sides.

In FIGS. 3B1 and 3B2, when viewed in cross section, a motor frame 3 b isformed into the irregular octagonal shape, and a magnet 2 b is formedinto the shape in which the thickness ratio of L1:L2 falls within therange of 1:2 to 1:3, and the straight sides are left. The otherrequirements are made identical to those of FIGS. 3A1 and 3A2.

In FIGS. 3C1 and 3C2, when viewed in cross section, a motor frame 3 c isconfigured by a combination of straight lines and arcs, and a ratio ofL1:L2, that is, a ratio of a radial thickness L1 of a portion (athinnest portion) where the magnetic poles are changed over to a radialthickness L2 of a central portion (a thickest portion) of the magneticpole is set in the range of 1:2 to 1:3, and moreover, a radius of thecorner R of a magnet 2 c that is constituted by an outer arc thereof isset to be in the range of 65% to 85% of an inner radius (inside radius)of the magnet 2 c.

These are the conditions for the case where the thickness in the radialdirection of the motor frame 3 c is thin to such an extent that nopractical problem is caused. However, in the event that the thickness ofthe motor frame 3 c causes a practical problem, a condition needs to beestablished with respect to the inner side of the motor frame 3 c whichhave conformable contact with the magnet 2 c. Alternatively, a conditionmay be set with respect to the outer side of the motor frame 3 c inconsideration of the thickness thereof.

In FIGS. 3D1 and 3D2, when viewed in cross section, a shape of the motorhaving 8 poles and 12 slots is shown, and is configured in a similar wayto those of the other examples. A motor frame 3 d has a shape showntherein in the same way as shown in FIGS. 3B1 and 3B2. A magnet 2 d isconfigured such that the 8 poles are disposed in corner portions of themotor frame 3 d, respectively and a ratio of L1:L2, that is, a ratio ofa radial thickness L1 of a portion (a thinnest portion) where themagnetic poles are changed over to a radial thickness L2 of a centralportion of the magnetic pole is set in the range of 1:2 to 1:3.

In FIGS. 3E1 and 3E2, when viewed in cross section, a motor frame 3 e isidentical to the one shown in FIGS. 3C1 and 3C2, and for each magneticpole of a magnet 2 e to be fixed independently, a configuration isadopted in which a gap 8 is provided between adjacent magnets. A ratioof L1:L2, that is, a ratio of a radial minimum thickness L1 of themagnet 2 e to a radial maximum thickness L2 at a central portion of themagnetic pole is set in the range of 1:2 to 1:3, and moreover, a radiusof the corner R of the magnet 2 e that is constituted by an outer arcthereof is set in the range of 65% to 85% of an inner radius (insideradius) of the magnet 2 e. By adopting this configuration, the samefunction and advantage as those of the small DC motor of the inventionin which no gap is provided between the adjacent magnets can beprovided.

A space in the gap is preferably provided in a range where the drivingproperty of the motor is not lowered.

(Embodiment 3)

FIGS. 6A and 6B illustrate sectional views of a coreless motor to whichthe magnet configuration of the invention is applied.

FIG. 6A is a sectional view taken along a line n-n in FIG. 6B, and FIG.6B is a transverse sectional view taken along a line m-m in FIG. 6A. Acoreless motor 29 of the invention is contained in the small DC motor ofthe invention.

The coreless motor 29 includes an armature assembly rotatably disposedwithin a motor frame 30, the motor frame 30 including a commutator mold37, an armature windings-molded element 38 and a shaft 7.

A motor frame 30 of the coreless motor 29 includes a cylindrical portion31, an end plate portion 32 which is provided continuously to thecylindrical portion 31, and a bearing support portion 33 which isprovided continuously to the end plate portion 32. In an interior of thebearing support portion 33, a shaft 7 is rotatably supported by twobearings 34, 35 which are separated from one another so as to beprovided at upper and lower ends of the bearing support portion 33,respectively. One end of the shaft 7 is brought into abutment with abottom plate 36. A commutator mold 37 is provided on the shaft 7.

The commutator mold 37 includes a cylindrical portion 37 a which is madeof resin and is provided in such a manner as to surround thecircumference of the bearing support portion 33 of the motor frame 30,an inner annular plate portion 37 b which is provided continuously to anend of the cylindrical portion 37 a and is fixed to the shaft 7, and anouter annular plate portion 37 c which is provided on the other end ofthe cylindrical portion 37 a in such a manner as to protrude radiallyoutwardly therefrom, and a commutator piece 39 a and a riser 39 b whichcontinues to the commutator piece 39 a to constitute a feeding path toan armature windings-molded element 38 are partially molded in thecommutator mold 37 in such a manner as to be embedded integrallytherewith. A radial outside of the riser 39 b electrically connects toand supports with a required strength the armature windings-moldedelement 38 via a tap 38 a made of a metallic elongated piece. A pair ofbrushes 40, which have contact with the commutator piece 39 a, aremounted on a terminal 41 fixed to the bottom plate 36.

The armature windings-molded element 38 takes a configuration in which acoil into which an armature winding is wound has a thin cylindricalshape and is then fixed by a resin, taking in the tap 38 a on its way tothe top so as to connect to the riser 39 b via the tap 38 a. An inneryoke 42 and field magnets 43 are disposed on a stationary side in such amanner as to hold the armature winding molded element 38 on a rotatingside therebetween. The inner yoke 42 includes a cylindrical portion 42 awhich faces the armature windings-molded element 38 and a bent portion42 b which is disposed on the bottom plate 36. The field magnets 43 aredisposed in such a manner that their magnetic centers are disposed tomatch connecting portions 45 which connect together individual sides 44of the motor frame 30.

The cylindrical portion 42 a of the inner yoke 42 short-circuitsmagnetic flux generated in the field magnets 43 to thereby reduceleakage magnetic flux.

The motor frame 30 includes the cylindrical portion 31, the cylindricalportion 31 having a constant thickness and having a cross section in ashape that comprises the four sides 44 and the connecting portions 45,each of the connecting portions 45 connecting adjacent two of the foursides 44 and being located inward from a corresponding corner in aquadrangle comprising the four sides 44, the end plate portion 32 whichis provided continuously to the cylindrical portion 31, and the bearingsupport portion 33 which is provided continuously to the end plateportion 32. In a cross section of the cylindrical portion 31, the sides44 that are separated from one another are connected to each other byarcs at the corner portions 45. The bearing support portion 33 has acylindrical shape.

The reduction in size of the motor can be enabled without reducing thethickness of central portions of magnetic poles of the field magnets 43by forming the shape of the cylindrical portion 31 of the motor frame 30basically into a quadrangular shape which has the same number of cornerportions as the number of magnetized magnetic poles of the field magnets43.

The outside diameter of the armature windings-molded element 38 is setrelative to the inside diameter of the cylindrical portion 31 of themotor frame 30 such that only a necessary minimum air gap G and a widthL1 at both ends of the field magnets 43 in the rotating direction areprovided therebetween at a position where the outside diameter of thearmature windings-molded element 38 comes nearest to the inside diameterof the cylindrical portion 31, that is, at a position of a centralposition of each side 44 in the event of this embodiment. In the exampleshown in FIG. 6, this setting is carried out at the central portion ofeach side 44.

The air gap G is determined mainly by accuracies of two constituentcomponents, that is, the accuracy of the inside diameter of the motorframe 30 and the accuracy of the outside diameter of the armaturewindings-molded element 38. Due to this, the air gap G takes a value ofthe order of 0.1 mm to 0.5 mm as an actual dimension.

The radius of the arc on an inside of the corner portion 45 of the motorframe 30 is set to any value in the range of 5% to 85% of a length fromthe center of the shaft 7 to an arc-shaped surface of an inside portion43 a of the field magnet 43 which lies on a shaft 7 side thereof,whereby the armature windings-molded element 38 can be disposed as beingenlarged in diameter without being restricted by the field magnets 43with respect to a location where it is disposed, and consequently, thewinding number of an armature winding can be increased so as to increasetorque to be generated. Preferably, the radius of the arc-shape at theconnecting portion 45 of the motor frame 30 is set to an arbitrary valuein the rang of 65% to 85% of the length from the center of the shaft 7to the arc-shaped surface of the inside portion 43 a of the field magnet43 which lies on the shaft 7 side thereof.

The field magnets 43 are made of, for example, neodymium magnets(Nd—Fe—B) or the like, are magnetized in a radial direction or rotatingdirection, and are disposed, respectively, at the connecting portions 45of the cylindrical portion 31 having the quadrangular cross section insuch a manner as to be spaced apart from one another.

The cross section of the field magnet 43 has a shape in which the insideportion 43 a (a side which lies conformable to the armaturewindings-molded element 38) exhibits an arc-shape having a radius fromthe center of the shaft 7, while an outside portion 43 b is made tofirmly secure to an inside surface 31 a of the cylindrical portion 31 ofthe motor frame 30. A connecting portion 43 c between the inside portion43 a and the outside portion 43 b has an angle at which it intersectsthe inside surface 31 a of the cylindrical portion 31 of the motor frame30 at right angles, but the connecting portion 43 c can also be has anarbitrary angle.

In particular, in the small DC motor 1, in order to optimize arelationship between the (BH) max (a maximum energy product) of thefield magnet 43 which largely affects the motor properties and theoutside diameter dimension of the armature windings-molded element 38which makes up a magnetic constituent portion of the rotating element,one or both of the following two conditions are set:

-   (1) A condition that the ratio of the width L2 to the width L1 of    the magnet is set in the range of 2.0≦L2/L1≦3.0; and-   (2) A condition that the ratio of corner R to the inside radius of    the magnet is set in the range of 65% to 85%.

The field magnets 43 may be such as to have portions which aremagnetized alternately to different magnetic poles in one directionalong the circumferential surface, or the magnetized portions of thefield magnets 43 may be separated from one another, or each of theportions of the field magnets 43 which are magnetized to differentmagnetic poles may have portions which have conformable contact with theconnecting portions 45 of the magnet frame 30.

By adopting this configuration, the effective magnetic flux of thearmature windings-molded element 38 can be increased, and furthermore, awinding region of the armature windings-molded element 38 can beincreased, and the quantity of the field magnets 43 to be used, whichare expensive as a motor component, can be suppressed, whereby torque tobe generated is increased while reducing in size of the motor, so thatthe cost and volume of the motor is reduced.

The configuration of the motor frame 30 and the field magnets 43 can bealtered variously as has been described above.

In the coreless motor of Embodiment 3, since the outside diameter of thearmature windings-molded element 38 is set relative to the insidediameter of the cylindrical portion 31 of the motor frame 30 such thatthe necessary minimum air gap G and the necessary minimum width L1 forthe field magnets 43 are provided therebetween at the position where theoutside diameter of the armature windings-molded element 38 comesnearest to the inside diameter of the cylindrical portion 31, that is,at the position of the central position of each side 44 in the event ofthis embodiment, the radial length of the armature windings-moldedelement 38 can be increased, and due to this, the winding space can beincreased, so that the diameter of the armature windings-molded element38 can be maximized to increase the starting torque, while minimizingthe quantity of the expensive field magnets 43 to be used, whereby thesize of the motor can be reduced.

In addition, since the shape of the cylindrical portion 31 of the motorframe 30 is formed into the quadrangular shape which has the same numberof connecting portions 45 as the number of magnetized magnetic poles ofthe field magnets 43, the size of the motor can be reduced withoutreducing the thickness of the central portions of the poles of the fieldmagnets 43 which accomplishes the sinusoidal magnetization properties.

Additionally, since the shape of the commutator mold 37 includes thecylindrical portion 37 a which is provided in such a manner as tosurround the bearing support portion 33 of the motor frame 30, the innerannular plate portion 37 b which is provided continuously to the end ofthe cylindrical portion 37 a and is fixed to the shaft 7, and the outerannular plate portion 37 c which is provided on the other end of thecylindrical portion 37 a in such a manner as to protrude radiallyoutwardly therefrom, the two bearings 34, 35 can be provided in such amanner as to be spaced apart from one another for stable support whilesecuring a contact space between the commutator piece 39 a and thebrushes 40, so as to allow the cylindrical portion 42 a of the inneryoke 42 to be formed long in the axial direction.

A resin reinforcement film can be provided on outside surfaces of theouter annular plate portion 37 c and the armature windings-moldedelement 38 so as to make up an vibration-proof construction. The tap 38a and the riser 39 b are connected together by welding. The tap 38 a hasa strip-shape and a curved portion is provided on part thereof in orderto impart elasticity.

Since the field magnets 43 and the inner yoke 42 are provided to faceone another while holding the armature windings-molded element 38therebetween, a magnetic path (the inner yoke 42 or the like) of amagnetic material can be provided long, and the magnetic resistance ofthe magnetic path can be suppressed to a lower level, thereby making itpossible to suppress the reduction in magnetic flux density. Inaddition, needless to say, cogging torque can also be suppressed.

(Embodiment 4)

FIGS. 7A and 7B illustrate sectional views of a slotless cored motor towhich the magnet arranging configuration of the invention is applied.

FIG. 7A is a sectional view taken along a line p-p in FIG. 7B, and FIG.7B is a transverse sectional view taken along a line o-o in FIG. 7A. Theslotless cored motor of this embodiment is also included in the small DCmotor of the invention.

A slotless cored motor 47 of the invention is configured such that, inplace of the inner yoke 42 of the coreless motor described in Embodiment3, a movable back yoke 46 is provided in such a manner as to be incontact with an inner side of an armature windings-molded element 38 andthe movable back yoke 46 is supported by a commutator mold 37. Themovable back yoke 46 is made of resin into which a magnetic material ismixed, or a magnetic material, has no slots and a cylindrical shape likethe armature windings-molded element 38.

Field magnets 43 are configured with four magnetic poles as inEmbodiment 3 and comprises a pair of N pole magnet 43MNL and S polemagnet 43MSL, and a pair of N pole magnet 43MNS and S pole magnet 43MSS.This embodiment is characterized by utilization of one pair of N polemagnet and S pole magnet which have the same magnetic flux density as aunit. Magnets having different magnetic flux densities can be combined.The magnets are arranged so as to be disposed relative to the center ofa shaft (rotating shaft) 7 at an even divergent angle.

To indicate a magnetic flux permeation path, for example, a magneticflux generated from the field magnet 43MNL having the N pole permeatesby way of the path via the armature windings-molded element 38, themovable back yoke 46, the armature windings-molded element 38, the fieldmagnet 43MSL having the S pole and a motor frame 30 in this order. Sincea location where the magnetic resistance becomes high is situated onlyat gaps among the magnet NML and the magnet MSL and the armaturewindings-molded element 38 along this path, a motor can be configuredwhich has large magnetic flux density and large torque compared to thecoreless motor in Embodiment 3.

In addition, since the movable back yoke 46 is provided inside thearmature windings-molded element 38, compared to the coreless motor inEmbodiment 3, inertial force becomes large and smooth rotation isprovided.

While the embodiments have been described as being applied to the innerrotor motors, the invention can also be configured as an outer rotormotor in the event that a shaft is provided on a motor frame in whichfield magnets are provided, windings are fixed thereto and drivingcurrent is made to flow through the windings. In addition, the inventioncan also be configured as a generator in the event that an externalpower is imparted to the shaft for rotation.

The configurations that have been described heretofore can be combineddifferently in an appropriate fashion without altering the functions.

The armature assembly is allowed to rotate both inside and outside themagnets which are arranged at a predetermined diverging angle.

In addition to the embodiments that have been described heretofore, theindividual configurations can be combined to configure small DC motorshaving arbitrary properties.

The present application claims foreign priority based on Japanese PatentApplication (JP 2005-156248) filed May 27 of 2005 and Japanese PatentApplication (JP 2005-348016) filed December 1 of 2005, and the contentsof which is hereby incorporated herein by reference.

1. A motor comprising: (a) a motor frame having an open end at one endthereof and an end plate portion at other end thereof, the motor framebeing a magnetic material and being a magnetic constituent portion ofthe motor; (b) a magnet disposed on an inner surface of the motor frame,wherein the magnet includes at least four poles and has an outer surfaceconforming to the motor frame and an inner surface forming a circularopening and wherein the width of the magnet between the inner and outersurface varies; (c) a rotor having a shaft and rotatably disposed in thecircular opening of the magnet; and (d) an end cap attached to the openend of the motor frame, (e) wherein the motor frame surrounds the magnetand includes four straight portions and at least four connectingportions, (f) wherein each of the connecting portions is located betweenadjacent two of the four straight portions, (g) wherein the motor frameincludes the same number of connecting portions as the number of polescontained in the magnet, (h) wherein an air gap provided between theinner surface of the magnet and the rotor has a constant distance, (i)wherein the magnet has a maximum thickness portion at portionscorresponding to the connecting portions of the motor frame and aminimum thickness portion gradually decreasing from the maximumthickness portion toward circumferentially opposite end, a ratio of themaximum thickness portion to the minimum thickness portion in a radialdirection is 2.0 or more, and (j) wherein the connecting portion is asegment of a circle having a center away from a center of the circularopening so that a ratio of a radius of the connecting portion to aradius of the circular opening is set to 85% or more.
 2. A motoraccording to claim 1, wherein the ratio of the maximum thickness portionto the minimum thickness portion in the radial direction is set in arange of 2.0 to 4.0.
 3. A motor according to claim 1, wherein the endplate portion is integrally formed with the motor frame as one piece. 4.A motor according to claim 1, wherein the end plate portion has anopening through which the shaft is passed.